1. Field of the Invention:
The present invention relates to a pulse width luminance modulation system for the display of a picture including half tone scenery on a dc gas discharge display panel having a working time lag.
2. Description of the Prior Art:
When a gas discharge display panel is used for a picture display, the operation time allotted for each discharge cell forming the display panel is generally limited to a certain value. In the following explanation this certain value is defined by H. The operation time used herein means to include the discharge duration of a discharge cell for making a continuous glow discharge and a preparatory time between application of the operation voltage to the discharge cell and the commencement of the discharge. As an example, we may consider a case of picture display by an NTSC standard television signal, in which all unit discharge cells forming one row of the discharge panel are excited simultaneously and the display panel is scanned vertically at a rate synchronized with the horizontal scanning period H.sub.TV of the NTSC television signal. In this case, an allowable operation time for one unit discharge cell is given by the following. EQU H = H.sub.TV = 63.56 .mu.s
Generally, there is a time lag, i.e., a working time lag, in a discharge phenomenon after application of the operation voltage and before starting the continuous glow discharge. Such time lag shows substantial deviation. However, when the discharge cell is supplied with charged particles from others as is the case of the so-called self-scan type panel, the deviation of said time lag decreases considerably and at the same time the breakdown voltage causing breakdown discharge in the gas decreases and it tends to approach the glow discharge voltage, which is maintained during the successive discharge period. The luminance modulation system according to the present invention is mainly concerned with a gas discharge display panel of the type in which charged particles are supplied by some means before starting the discharge.
In the conventional pulse width luminance modulation system in which the discharge time after starting the discharge is modulated, the pulse width W of the driving pulse should be larger than the time lag T before the starting of the glow discharge. By assuming that the time other than the time lag for starting the discharge consists of the discharge duration (W-T), the pulse width W should be modulated in a following range: EQU 0 .ltoreq. W .ltoreq. H--T
on the other hand, the maximum value I.sub.max of the discharge current is decided by the construction of the discharge cell, composition of the gas, pressure of the gas, etc. and it can not be made larger. Therefore, a highest brightness of the luminance is given by: EQU I.sub.max.sup.. (H-T).sup.. fv
This luminance value can not be made higher and fv is the repetition frequency of radiation of a discharge cell. In the abovementioned embodiment of the NTSC television signal, the value fv corresponds to the field frequency of about 60 Hz.
As explained above, there is a certain maximum limit for the brightness obtained by a discharge cell. Accordingly, in order to obtain a large contrast ratio, it is required to give an ability to display luminance as small as possible. However, the brightness of the discharge luminance does not decrease in proportion to the ratio of shortening of the discharge time. This is also applied to a case when the driving voltage is discontinued immediately after the commencement of the discharge, in which the discharge time (W-T) is nearly equal to 0. By this reason a desireable sufficient contrast ratio has been difficult to obtain only by applying a conventional pulse width luminance modulation.